1. Field of the Invention
The present invention relates to a tracking control device and, more particularly, to a device capable of controlling the relative position between a reproducing head and a recording medium during the reproduction of a digital information signal from a track formed on the recording medium.
2. Description of the Related Art
Reproduction tracking systems for domestic-use video tape recorders or video cassette recorders fall into two major systems. The first system is called "CTL system". In the CTL system, a fixed magnetic head is provided at a particular location in a tape path. During recording, a control signal (CTL signal) is recorded on a dedicated track, and, during reproduction, a reproducing head is made to trace a track by using the CTL signal reproduced from the dedicated track. The second system is called "ATF system". In the ATF system, during recording, a plurality of (normally, four) kinds of pilot signals having comparatively low frequencies are recorded simultaneously with a main signal (normally, a video signal) in multiplexed form in such a manner that the frequencies are recurrently repeated from track to track. During reproduction, a tracking error signal (ATF error signal) is obtained by making a comparison between the frequency of the pilot signal reproduced from a track simultaneously with the main signal and the pilot signals reproduced from both adjacent tracks by crosstalk, and a reproducing head is made to trace the track on the basis of the tracking error signal.
In recent years, higher image quality and digitization have been increasingly demanded even in the field of domestic-use video tape recorders, and research and development have been conducted as to formats of narrower track pitch so that more information can be recorded and reproduced. In the case of reproduction from a narrow-pitch track, it is extremely important to increase the accuracy of the longitudinal linearity of the track. For example, in the case of a track of 10-.mu.m pitch, the linearity must be held at 3 .mu.m or less. This value is close to the limit of mechanical precision and offers great problems in terms of productivity and cost.
In general, the relationship between reproduction RF level and error rate of reproduced data in the case of a digital VTR is represented by the characteristic shown in FIG. 1. In general, in digital recording, if the reproduction RF level is not less than a particular level, the quality of reproduced signals is not substantially influenced. For example, as shown in FIG. 1, if the reproduction RF level is not less than a proper threshold (for example, 50%), the error rate of the reproduced data will show a low value which is a generally tolerable error rate (for example, a threshold Eth of approximately 10.sup.-4). If the reproduction RF level becomes less than the threshold Eth, the error rate increases sharply. In the case of the characteristic shown in FIG. 1, the reproduction RF level may be not less than 50%.
There is also a known art (DTF system) provided with an actuator for displacing a rotary head in a direction which intersects the rotating direction thereof. According to this art, even if a track is curved to some extent, the rotary head can be made to trace the curvature. Although the art is effective in improving tracking performance, it is necessary to use an extremely expensive rotary drum having a very complicated structure. In particular, it is extremely difficult to incorporate such an actuator into a small-diameter rotary drum, and complicated and awkward position control of the rotary head is needed.
Tracking control signals, such as the tracking error signal used in the ATF system, are normally formed by quantifying the amount of deviation of tracking. As a result, if tracking is controlled (i.e., the rotation of a capstan is controlled) by using only such a tracking control signal, the tracking control will result in average-value control which may not necessarily be effective in reducing the error rate of reproduced data in the digital VTR. In such a case, the image quality of reproduced images is not improved.